1. Technical Field
The disclosure relates to image sensors or imagers in CMOS technology.
2. Description of the Related Art
FIG. 1 is a schematic sectional view of a pixel of a CMOS image sensor. In this figure and the following, the different pixel elements are shown with dimensions chosen to make the figures intelligible and are not drawn to scale. The doping levels of the P-conductivity type zones are shown with shades of gray that are all the darker as the doping levels increase.
The image sensor is formed in an active layer 10, usually of P conductivity type, having a doping level noted P−. The layer 10 is formed on a substrate 12, often of P− conductivity type. The doping level of the substrate, denoted by P+, is generally much higher than the doping level of the active layer 10, typically 3 decades. The layer 10 may have a thickness between 3 and 6 microns, while the substrate may have a thickness of 780 microns.
A buried layer 14 of N-conductivity type, close to the upper face of the layer 10, forms a photodiode with the layer 10. As shown, the portion of the layer 10 above the zone 14 may have a higher doping level than the layer 10 to provide a passivation of the top interface. The upper surface of the layer 10 bears various conductive zones for controlling the pixel, including a transfer gate TG. These conductive zones and other metal tracks may be embedded in a passivation layer 16.
The pixel may be laterally isolated from its neighboring pixels by trench isolators 18, typically including semiconductor oxide, which extend throughout the thickness of the active layer 10. Alternatively, the insulation between the pixels may be achieved by an over-doping (P-type) relative to the layer 10, but such insulation is known to be less effective from both an electrical and an optical point of view. The spacing between the trench isolators 18 defines the size of the pixels.
In the case of a color sensor, individual color filters 19 are attached to the layer 16 in correspondence with the pixels. The filters 19 usually bear individual collimating lenses 20.
In operation, during an integration phase, the photodiode 10/14 is floating and photons absorbed by the active layer generate electrons that are stored in the zone 14. At the end of the integration phase, the stored charge represents the amount of light received by the pixel.
A recurring problem with this pixel structure is that electrons reach into the active layer 10 even in the absence of light, producing a so-called dark current. The dark current being different between pixels, or from one integration phase to another, the image captured by the sensor has exposure differences from one pixel to the other, even if the pixels have received the same amount of light. This phenomenon produces a visible noise in images captured in low light conditions.
The origins of dark current are not well known. An identified origin is the presence of imperfections or impurities in the active layer that promote the generation of electrons that diffuse towards the junction of the photodiode.
To limit this phenomenon, interfaces that may present a poor surface state are neutralized, such as the interface between the trench isolators 18 and the layer 10. As shown, a P-type layer having a higher doping level than the active layer 10 may line the trench isolators 18. The doping level may be the same, P+, as the substrate. Thus, the generation centers are less active and there are fewer electrons that may diffuse to the zone 14.
Despite these measures, a dark current remains.